Solar energy concentrator and converter

ABSTRACT

A solar energy concentrator is formed in the shape of a spiral horn, so that solar energy incident over a wide range of incident angles on the mouth of the horn is concentrated by multiple reflections from inner walls of the horn to emerge from an exit aperture at the centre of the horn. Solar energy emerging from the collector may be distributed by light pipes to illuminate a building or may be transmitted to a solar energy conversion chamber having a small entry aperture. The entry aperture acts as black body absorbing all solar energy incident upon it and the solar energy may be converted within the chamber either by photovoltaic cells and/or by heat absorbing media.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a Continuation of U.S. patent applicationSer. No. 09/830,764, filed Aug. 6, 2001, entitled “SOLAR ENERGYCONCENTRATOR AND CONVERTER” which is a US National Phase ofInternational Patent Application serial No. PCT/GB99/03757, filed Nov.11, 1999 which claims the priority of Great Britain Patent Applicationserial No. 9824771.1, filed Nov. 11, 1998.

BACKGROUND OF THE INVENTION

[0002] This invention relates to a solar energy concentrator, a solarenergy concentrator in combination with a solar energy distributor and asolar energy concentrator in combination with a solar energy conversionchamber.

[0003] Solar energy concentrators, for use prior to converting solarenergy into other useful forms of energy, are well known. Concentratedsolar energy may be used in a solar furnace or converted into otherforms of energy by, for example, a thermally absorbent medium, usuallycontaining a fluid, or by a photovoltaic cell.

[0004] Known solar concentrators include parabolic dish receivers which,in order to focus direct sunlight, track the sun across the sky. Thesedevices are 1usually built on towers which must be able to withstandsubstantial wind shear while producing a minimum of shadow on the faceof the collector. The shape of the collector must remain constant overtime, and tracking must be accurate to maintain an angle of incidencewithin one degree throughout the day. Further, when used in conjunctionwith a Stirling engine there are also potential problems of wind gustscreating fluctuations in the heat exchange and therefore in the poweroutput.

[0005] Land use (8-17 acres per megawatt, 3.2-6.9 hectares/megawatt),site preparation, installation, capital costs and maintenance ofheliostats in the towers are also expensive. Solar concentrators, whichuse either a lens or a compound reflective surface, are alsocommercially available but they too require tracking mechanisms to trackthe sun's movement across the sky. The cost of purchase, installation,maintenance and associated land requirements are again substantial.

[0006] Various solar concentrators are also known, which do not requiretracking mechanisms, however, they suffer from various limitations,e.g., the acceptance angle of some of the concentrators are so limitedthat the sun's rays can be received only for a small portion of the day.In others, a concentration factor may be satisfactory only during alimited time of day.

[0007] Moreover, known concentrators are designed for operation indirect sunlight, and do not function satisfactorily in diffuse orscattered light. For example, the solar collector of U.S. Pat. No.4,287,880 (Geppert) comprises a reflector formed from three separatecurves, which focus solar energy onto a pipe collector, such that raysof sunlight having different angles of incidence are reflected bydifferent parts of the reflector onto the collector pipe to heat a fluidflowing through the pipe. However, the device cannot effectivelyconcentrate diffuse light and is therefore limited in its geographicalapplication. The efficiency of the pipe absorber will also vary as afunction of the ambient air temperature, because heat absorption,transfer and collection is external to the device.

[0008] An involute beam concentrator disclosed in U.S. Pat. No.4,610,518 (Clegg) uses an involute chamber to convert a concentratedrectangular beam of sunlight emergent from a prismatic beam concentratorinto a concentrated solar beam parallel to an axis of the concentrator.This concentrator is designed to accept input solar energy from theprismatic beam collector only over a very small range of incidentangles.

[0009] The use of photovoltaic cells in the form of silicon solar cellsalso suffers from the disadvantage that the spectral distribution ofsunlight has a maximum spectral radiance at a wavelength of 540nanometres whereas the maximum sensitivity of the solar cells occurs at813 nanometres. As a result, much of the energy falling on the solarcell is not converted into electricity. Moreover, these solar cells haveto be cooled to maintain operational peak efficiency.

[0010] U.S. Pat. No. 4,947,292 (Vlah) discloses a lighting systemdesigned to produce a diffuse light from a concentrated light source, inwhich a concentrated light source is located at the focus of aspiral-shaped horn and diffuse light is emitted from the mouth of thehorn. A preferred shape of the spiral is a “Golden Section spiral”, alsoknown as a volute, formed from a series of nested “Golden Section”rectangles, i.e., rectangles in which the ratio of the lengths of thelarger and smaller sides is ½(1+5^(1/2)):1, which may be used to locatethe “Golden Section” centres for the arcs which make up a “GoldenSections” spiral.

SUMMARY OF THE INVENTION

[0011] It is an object of this invention to provide a solar energyconcentrator, a solar energy concentrator in combination with a solarenergy distributor and a solar energy concentrator in combination with asolar energy conversion chamber which at least partially mitigate someof the difficulties of the prior art.

[0012] According to a first aspect of the invention there is provided asolar energy concentrator comprising a spiral horn having an axisperpendicular to a plane of the spiral, said concentrator including: aninput aperture forming a mouth of the horn, an internal light-reflectingsurface of the horn, and an exit aperture at an end of the horn remotefrom the mouth of the horn, said exit aperture being smaller than saidinput aperture and said horn continuously 1tapering both in thedirection of said axis and in the plane of the spiral, between the inputand output apertures, wherein the horn is adapted to concentrate, bymultiple reflections from the internal light-reflecting surface of thehorn, solar energy incident within a predetermined range of angles ofincidence on the input aperture, such that concentrated solar energy isemitted from the exit aperture.

[0013] Preferably at least one director is provided in the mouth of thehorn to reflect light incident from outside the predetermined range ofangles of incidence into the predetermined range of angles of incidence.

[0014] Conveniently, the at least one director is a baffle disposedsubstantially parallel to the axis of the spiral horn.

[0015] Advantageously, the at least one director is a partial spiralhorn disposed substantially perpendicular to the axis of the spiral hornin at least a portion of the spiral horn most proximate to the mouth ofthe horn.

[0016] Conveniently, the spiral horn has a substantially quadrilateralcross-section parallel to the axis of the horn.

[0017] Advantageously the taper in the plane of the spiral is a GoldenSpiral.

[0018] Conveniently the horn is of metal.

[0019] Advantageously the metal is aluminium.

[0020] Preferably the horn has portions formed of different materialsdisposed along the horn spiral, the materials being adapted to withstandthe temperatures reached in the respective portions of the collector inuse.

[0021] Advantageously a portion of the horn proximate the exit apertureis of a ceramic material.

[0022] Conveniently the light-reflecting surface is protected byultraviolet radiation absorbing means.

[0023] According to a second aspect of the invention there is provided asolar energy concentrator according to the first aspect in combinationwith distribution means in communication with the exit aperture andadapted for distributing the concentrated solar energy emitted from theexit aperture.

[0024] Preferably the distributions means includes at least one lightpipe.

[0025] Advantageously the distribution means includes a diffuser fordiffusing at least some of the concentrated solar energy to provideillumination.

[0026] Conveniently the diffuser is in the shape of a spiral horn.

[0027] According to a third aspect of the invention there is provided asolar energy concentrator according to said first aspect, in combinationwith a solar energy conversion chamber having a chamber aperture incommunication with the concentrator exit aperture, the chambercontaining energy conversion means for converting concentrated solarenergy emitted from the exit aperture.

[0028] Advantageously, the energy conversion means includes aphotovoltaic cell. Conveniently the energy conversion means includesheat absorbing media.

[0029] Advantageously the energy conversion means includes steamgenerating means.

[0030] Conveniently, the energy conversion means includes a solarfurnace.

[0031] Advantageously, at least some of the solar energy is reflectedwithin the chamber before being incident on the energy conversion means.

[0032] Advantageously at least some of the solar energy undergoeswavelength changes within the chamber.

[0033] Conveniently, the solar energy undergoes wavelength increases by1energy absorption and/or dissipation.

[0034] Conveniently, solar energy distribution means is provided totransmit solar energy from the exit aperture to the chamber aperture.

[0035] Advantageously the distribution means includes at least one lightpipe.

[0036] The first aspect of the present invention has the advantage thatthe collector can collect solar energy over a large range of angles ofincidence without the need for tracking mechanisms. The collectortherefore efficiently collects and concentrates diffuse light. Anadvantage of the third aspect of the invention is that the chamberaperture of the solar energy conversion chamber approximates a blackbody so that most of the energy entering the chamber is absorbed withinthe chamber. The wavelength of solar energy may also be changed in thechamber to enable more of the energy to be absorbed by a photovoltaiccell and converted into electricity. In addition, any heat produced mayalso be utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The invention will now be described by way of example withreference to the accompanying drawings, in which

[0038]FIG. 1 shows a solar energy collector according the invention,

[0039]FIG. 2 shows a side wall of the collector of FIG. 1 before beingwound into a spiral,

[0040]FIG. 3 shows a cross-section of a collector of the invention.

[0041]FIG. 4 shows a cross-section of a collector of the inventionhaving baffles in the mouth of the horn,

[0042]FIG. 5 shows a light pipe used in the invention, and

[0043]FIG. 6 shows a conversion chamber of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0044] In the figures like reference numerals denote like parts.

[0045] The solar collector 1 shown in FIG. 1 is a spiral horn, having aninput aperture 2 and an exit aperture 3 in a first plane. The lower edge4 of an inner wall 5 and the lower edge 6 of an outer wall 7 define asecond plane 8 perpendicular to the first plane and the inter-section ofthe lower edge 6 of the outer wall 7 with the second plane 8 and theinter-section of the lower edge 4 of the inner wall 5 with the secondplane 8 lie on a same “Golden Section” spiral, as indicated by theprojection of the inter-section of the lower edge 6 of the outer wall 7and the inter-section of the lower edge 4 of the inner wall 5 shown bydotted lines 9 in FIG. 1. The inner wall 5 and outer wall 7 before beingformed into a spiral have the shape shown in FIG. 2. The transversecross-section of the spiral is a quadrilateral, the inner wall 5 beingshorter than the outer wall 7 such that the upper surface 10 of the hornas shown in FIG. 1 is angled in towards the centre 11 of the spiral. Asbest shown in the FIG. 2, the height of both the inner wall 5 and theouter wall 7 decreases from the input aperture 2 to the exit aperture 3.

[0046]FIG. 3 shows a cross-sectional view of the spiral horn of FIG. 1.1The shape of the spiral may be plotted using the function rkfixedavailable, for example, in the computer package “Mathemitca” availablefrom Wolfram Research Inc., PO Box 6059, Champaign, Ill. 61821-9902,USA, or in the package “Mathcad” available from Mathsoft Inc, 101 MainStreet, Cambridge, Mass., 02142 USA.

[0047] An incident ray 20 as shown by a broken line in FIG. 3, undergoesmultiple reflections on the internal surface of the outer and innerwalls 7, 5 of the spiral horn 1 to be directed towards the centre orfocus 11 of the spiral horn 1. However, a ray of light 27 incident atthe same point 23 as the ray 20 on the spiral at a smaller angle ofincidence undergoes multiple reflections and is reflected out of theinput aperture 2 of the horn 1.

[0048] With the insertion of baffles 30 in the mouth 2 of the horn 1, asshown in FIG. 4, a ray of light 31, as shown by the broken line,incident on the same point 23 of the spiral horn as shown in FIG. 3 isreflected by the baffle 30 towards the wall 7 of the spiral horn 1 andis reflected ultimately towards the centre 11 of the spiral. Inaddition, a ray of light 32, which in the absence of the baffle, wouldbe incident of the same point 23 on the spiral horn 1 at a smaller angleof incidence, as shown by the full line in FIG. 4, is incident insteadon the baffle 30 and reflected thereby onto the surface of spiral horn 1from where it is reflected towards the centre 11 of the spiral. Thus,the insertion of baffles 30 within the mouth of the horn increases therange of angles of incidence of solar energy rays which are reflectedtowards the centre 11 of 1the spiral. The baffles 30, as shown in FIG.4, also follow the curve of a spiral, thereby forming a number of spiralhorns within the mouth 2 of the horn of the collector.

[0049] Alternatively, instead of vertical baffles 30 located between alower surface 12 and the upper surface 10 of the mouth 2 of the spiralcollector, a plurality of partial spiral horns may be inserted in themouth 2 of the spiral horn to form a honeycomb of small spiral hornswithin a portion of the collector closest to the input aperture.

[0050] The spiral horn 1 may be formed of polished metal, for example,aluminium. However, because of the high temperatures reached towards theportion of the spiral closest to the exit aperture 3, a portion of thespiral horn closest to the exit aperture 3 may be formed of a ceramicsmaterial. Additionally, a portion of the horn closest to the inputaperture 2, which is subject to lower temperatures than the rest of thehorn may be formed of a plastics material coated with metal to form areflective surface.

[0051] The entry aperture 2 may be covered by a window to absorbultraviolet light, to protect the reflective surfaces of the spiral horn1 from damage from ultraviolet light, or preferably the horn may beformed of a glass-metal-glass sandwich in which the glass layers absorbultraviolet light.

[0052] Light exiting from the exit aperture 3 may be directed by meansof 1known light pipes 40, as shown in FIG. 5. As indicated in FIG. 5,light 41, 42 from more than one collector may be combined by a ‘Y’junction 43 of light pipes 40 for subsequent distribution.

[0053] Light exiting from the collector 1 may be used in any knownmethod of solar energy conversion, or may be distributed by light pipes40, to provide illumination, for example, in a building. Light may beextracted from the light pipe 40 for illumination, by means of adiffuser. The diffuser may be in a form of a spiral horn, wherein lightfrom the light pipe 40 is incident on a small input aperture at thecentre of a spiral horn and diffuse light is diffused from a large exitaperture at the mouth of the horn. That is, the diffuser may be a spiralhorn used in an opposite sense to that of the spiral horn collector. Itwill be understood that such a diffuser could be connected directly tothe solar collector 1 without the use of a light pipe 40.

[0054] Alternatively, a solar energy converter 50 may be provided asshown in FIG. 6. The solar energy converter comprises a cylindricalchamber having an input aperture 51 in the centre of one circular endface 52. The internal walls of the solar collector may be provided withphotovoltaic cells or with thermal energy absorbing means, such as pipescontaining a fluid for the absorption and conversion of solar energyincident through the input aperture 51. Solar energy entering the solarconverter via the input aperture 51 may undergo multiple reflectionswithin the chamber 50 before being absorbed. In addition, the inside ofthe chamber 50 may be provided with wavelength-converting means, for1example to convert the wavelength of the solar energy into a wavelengthmore suitable for absorption by photovoltaic cells. In order to maintainthe efficiency of the solar cells they may be cooled in a known manner,by, for example, a cooling fluid and the energy gained by the coolingfluid may also be utilized in a known manner.

[0055] It will be appreciated by the person skilled in the art that theshape of the chamber 50 is not critical and any convenient shape whichwill promote multiple reflections within the chamber 50 will besuitable. Similarly, the placement of the aperture 51 is not critical.The input aperture 51 of the solar collector may be connected directlyto the exit aperture 3 of the solar collector 1 or the solar collector 1may be connected to the solar converter 50 by means of light pipes 40.

What is claimed is:
 1. A solar energy concentrator comprising a spiralhorn having an axis perpendicular to a plane of the spiral, saidconcentrator including: an input aperture forming a mouth of the horn,an internal light-reflecting surface of the horn, and an exit apertureat an end of the horn remote from the mouth of the horn, said exitaperture being smaller than said input aperture and said horncontinuously tapering both in the direction of said axis and in theplane of the spiral, between the input and output apertures, wherein thehorn is adapted to concentrate, by multiple reflections from theinternal light-reflecting surface of the horn, solar energy incidentwithin a predetermined range of angles of incidence on the inputaperture, such that concentrated solar energy is emitted from the exitaperture.
 2. A solar energy concentrator as claimed in claim 1, whereinat least one director is provided in the mouth of the horn to reflectlight incident from outside the predetermined range of angles ofincidence into the predetermined range of angles of incidence.
 3. Asolar energy concentrator as claimed in claim 2, wherein the at leastone director is a baffle disposed substantially parallel to the axis ofthe spiral horn.
 4. A solar energy concentrator as claimed in claim 2,wherein the at least one director is a partial spiral horn disposedsubstantially perpendicular to the axis of the spiral horn in at least aportion of the spiral horn most proximate to the mouth of the horn.
 5. Asolar energy concentrator as claimed in claim 1, wherein the spiral hornhas a substantially quadrilateral cross-section parallel to the axis ofthe horn.
 6. A solar energy concentrator as claimed in claim 1, whereinthe taper in the plane of the spiral is a Golden Section Spiral.
 7. Asolar energy concentrator as claimed in claim 1, wherein the horn is ofmetal.
 8. A solar energy concentrator as claimed in claim 7, wherein themetal is aluminium.
 9. A solar energy concentrator as claimed in claim1, wherein the horn has portions formed of different materials disposedalong the horn spiral, the materials being adapted to withstand thetemperatures reached in the respective portions of the collector in use.10. A solar energy concentrator as claimed in claim 9, wherein a portionof the horn proximate the exit aperture is of a ceramic material.
 11. Asolar energy concentrator as claimed in claim 1, wherein thelight-reflecting surface is protected by ultraviolet radiation absorbingmeans.
 12. A combination of the solar energy concentrator as claimed inclaim 1 and distribution means in communication with the exit apertureand adapted for distributing the concentrated solar energy emitted fromthe exit aperture.
 13. A combination as claimed in claim 12, wherein thedistribution means includes at least one light pipe.
 14. A combinationas claimed in claim 12, wherein the distribution means includes adiffuser for diffusing at least some of the concentrated solar energy toprovide illumination.
 15. A combination as claimed in claim 14, whereinthe diffuser is in the shape of a spiral horn.
 16. A solar energyconcentrator as claimed in claim 1 in the combination with a solarenergy conversion chamber having a chamber aperture in communicationwith the concentrator exit aperture, the chamber containing energyconversion means for converting concentrated solar energy emitted fromthe exit aperture.
 17. A combination as claimed in claim 16, wherein theenergy conversion means includes a photovoltaic cell.
 18. A combinationas claimed in claim 16, wherein the energy conversion means includesheat absorbing media.
 19. A combination as claimed in claim 16, whereinthe energy conversion means includes steam generating means.
 20. Acombination as claimed in claim 16, wherein the energy conversion meansincludes a solar furnace.
 21. A combination as claimed in claim 16,wherein at least some of the solar energy is reflected within thechamber before being incident on the energy conversion means.
 22. Acombination as claimed in claim 16, wherein at least some of the solarenergy undergoes wavelength changes within the chamber.
 23. Acombination as claimed in claim 22, wherein solar energy increaseswavelength by energy absorption and/or dissipation.
 24. A combination asclaimed in claim 16, wherein solar energy distribution means is providedto transmit solar energy from the exit aperture to the chamber aperture.25. A combination as claimed in claim 24, wherein the distribution meansincludes at least one light pipe.